Feeding and digestion in the ectoparasitic copepod Lepeophtheirus salmonis (Kroyer, 1837)

Abstract

A study has been carried out to investigate some aspects of the feeding and digestion of the parasitic copepod Lepeophtheirus salmonis (Kreyer, 1837), a serious pathogen of wild and farmed marine salmonids. The alimentary canal consists of a cuticularised foregut and hindgut and a midgut, the latter comprising most of the length of the alimentary canal. It consisted of an anterior diverticulum, and anterior midgut, mid midgut and posterior midgut. All the midgut is lined in the luminal side with a monolayer of digestive epithelium. The latter is supported by a porous basal membrane. Mesenteries suspend the gut in the haemocoel and attach to the basal membrane by means of extensions of their cytoplasm that interlock with the pores of the basal membrane and are intimately associated with the cellular basal membrane of the enterocytes. Four types of cells could be recognised ultrastructurally and were named according a widely accepted nomenclature for the types of digestive epithelial cells found in decapod crustaceans. R-cells were the most abundant type and were believed to be involved in the absorption of nutrients, storage of lipids and probably excretion of waste material. Once the R-cells have finished their useful life, it is proposed that they finally die after passing through a stage, called A-cell stage, comparable to apoptosis or programmed cell death, after which the A-cell is finally sloughed off the epithelium and discarded in the faeces. F-cells constituted the second cell type, in charge probably of the secretion of digestive enzymes. Once this stage is finished, F-cells are believed to transform into the third type of cells, the B-cells, which engage in intracellular digestion of nutrients. At the end of their useful life they are sloughed off the epithelium. The last cell type, the E-cells, are believed to be embryonic cells that differentiate either into R- or F-cells. A revised mode of feeding is proposed, in which the louse scoops up strips of host skin epithelium of relatively constant size and shape, by means of the structures associated with the mouth cone. Analysis of the gut contents and the faecal pellets revealed that the main food item was host skin epithelium. The feeding activity usually reaches the dermal layer, suggested by the common occurrence of host melanin in the gut and in the faecal pellets. Two types of faecal pellets were found. The first type was a cylindrical one which was made of the ingested host's epithelial cells with little signs of having being digested. Cylindrical pellets could be clear or dark depending on the amount of host melanin present in them. The second type of pellet was a tape- or ribbon-like gelatinous pellet produced mainly when the lice were not eating (as during starvation) or sometimes associated to the end of the cylindrical pellets. Pellets were always surrounded by a delicate peritrophic envelope composed of several peritrophic membranes. Blood was believed to be another important food item. Blood feeding was commonly associated with the production of lipids in the R-cells and it is proposed (but not confirmed) that this alternative food item could be associated with some physiological process that requires lipogenesis, like vitellogenesis. A method for manually feeding the lice was devised. Salmon blood and milk were fed in this way. No sign of digestion of these food items was observed suggesting that possibly an endogenous factor different from the presence of food in the gut is necessary to trigger the digestive process. An analysis of type of gut contents, peritrophic membranes and cell type occurrences during a digestive period was carried out. The results suggested that some digestive processes take place preferentially in one or another gut region. Mid and posterior midgut are probably associated with the production of faecal pellets, production of digestive enzymes, intracellular digestion and excretion of waste materials judging by the higher occurrence of peritrophic membranes, B- and A-cells in these regions. Absorption of nutrients probably takes place throughout the midgut. The morphological changes in the topography shown by the midgut epithelium suggest that it is a very active and plastic tissue. The results of this study were discussed and compared with similar studies of other crustaceans, including other parasitic and free living copepod species. Suggestions were made of the implications of the present findings in the search for a control method for this parasite.